Re: FailedAssertion on partprune

On 17 August 2018 at 06:52, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> I don't know whether there's actually a defect here any more. I was
> trying to dispel some perceived confusion on the part of David and Tom
> about what this code was trying to accomplish, but the fact that the
> code caused some confusion does not necessarily mean that there's
> anything wrong with it. On the other hand, perhaps it does have
> functional deficiencies, or perhaps the comments need work. Or maybe
> this code is fine taken in isolation but there are still problems in
> how it interacts with partition pruning. Let's start by deciding what
> we think the problems are, and then we can decide who should fix them.

I think Tom and I both agree that the plan mentioned in [1] and [2] is
not quite as optimal as it could be. The investigation I did in [3]
highlights where this got broken and the reason why it got broken.

Tom removed the offending Assert() in 59ef49d26d2, but mentioned
there's no test case which tries to ensure having a partitioned table
as an Append child works correctly. I agree that it would be nice to
have a test for this, but I did try a couple of times to come up with
a test case to do this, but I was unable to come up with anything
suitable compact for the regression tests, and even at that, given how
difficult it is to get a plan into this shape, I didn't hold up much
hope of the test's plan remaining stable. Tom seems to agree with this
as he mentioned in [2].

I think the best path forward is if you (Robert) could verify the
behaviour that I mentioned in [3] is expected behaviour, if it is then
perhaps the path forward is for me to try harder to write a compact
test for this, but if you discover that the behaviour is unexpected
then I think the best course of action is to fix it to properly
flatten the
Append which will mean there's no need for a test case, and perhaps
the Assert that was removed can be put back again.

[1] https://www.postgresql.org/message-id/CAKJS1f9MQd5ntg6xXg7jJe0jB_bWvCDRmaH6yNzZGF%2BTVa5M%3DA%40mail.gmail.com
[2] https://www.postgresql.org/message-id/8600.1533765148%40sss.pgh.pa.us
[3] https://www.postgresql.org/message-id/CAKJS1f_j9BSJ7svDF7uid6EMm%2Bfu%2BcAhonZ7hcqiYU4ssv3rtg%40mail.gmail.com

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David Rowley http://www.2ndQuadrant.com/
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